The effects of sub-critical and super-critical carbon dioxide adsorption-induced coal matrix swelling on the permeability of naturally fractured black coal

Swelling of the coal matrix with the adsorption of CO₂ is one of the leading problems for CO₂ sequestration in deep coal seams as it causes coal seam permeability to be significantly reduced. The main objective of this study was to investigate the effect of coal mass swelling on the permeability of naturally fractured black coal. A series of permeability tests were conducted using a newly developed tri-axial apparatus on 38 mm by 76 mm naturally fractured black coal specimens. These tests were carried out for CO₂ and N₂ injections at 2-20 MPa injection pressures under 10 to 24 MPa confining pressures at 33 °C. Each coal specimen was then allowed to swell under sub-critical and super-critical CO₂ adsorption and the corresponding effects on CO₂ and N₂ permeabilities were examined. Results indicate that the permeability of naturally fractured black coal is significantly reduced due to matrix swelling, which starts as quickly as within 1 h of CO₂ injection. A further reduction is then observed, and the maximum swelling rate occurs within the first 3-4 h of CO₂ adsorption. The amount of coal matrix swelling due to CO₂ adsorption clearly depends on the phase condition of the CO₂, and super-critical CO₂ adsorption-induced swelling is about two times higher than that induced by sub-critical CO₂ adsorption. Interestingly, although a fractured coal specimen which has already fully swelled under sub-critical CO₂ adsorption can swell significantly more under super-critical CO₂ adsorption, after swelling under super-critical CO₂ adsorption, no further swelling effect occurs under any CO₂ pressure or phase condition. Moreover, the swelling process continues longer under super-critical CO₂ adsorption. It is concluded that super-critical CO₂ adsorption can induce more matrix swelling than sub-critical CO₂ adsorption under the same adsorption pressure.